The present invention provides a stable, efficient, single-ply, immobilized liquid membrane comprising an aqueous liquid membrane immobilized within a hydrophobic microporous support, and a method of preparing such an immobilized liquid membrane. The present invention also includes a method of preparing an ultrathin immobilized liquid membrane having a thickness of about 0.0084 mm or less.
The removal of a gaseous component from a gaseous mixture by an immobilized liquid membrane is well-known. Typically, such conventional immobilized liquid membranes are prepared by immersing a hydrophilic microporous membrane in a suitable aqueous solution. The hydrophilic membrane draws up the aqueous solution into its pores such that the solution in the pores acts as membrane so as to preferentially separate a gaseous component from a gaseous mixture. That is, one of the species in a gaseous mixture preferentially permeates through the liquid in the pores.
The conventional immobilized liquid membrane consists of an aqueous liquid membrane immobilized within a hydrophilic microporous membrane. The immobilized liquid membrane is then supported on a hydrophobic microporous support membrane on the lower pressure side to prevent expulsion of the immobilized aqueous liquid serving as the membrane. Examples of such a sandwich structure may be found in U.S. Pat. Nos. 3,819,806; 4,089,653; 4,115,513; 4,119,408; 4,147,754 and 4,174,374. When flat microporous membranes are used and a positive pressure difference exists between the two sides of the immobilized liquid membrane, the sandwich structure described above is further supported by a flat, fine mesh stainless steel screen. See, Kimura and Walmet, "Fuel Gas Purification With Permselective Membranes", Separation Science and Technology, 15 (4), p.p. 1115-1133 (1980). When hydrophilic microporous hollow fiber support membranes are used to immobilize aqueous liquids, the application of a positive pressure difference is avoided to prevent the expulsion of the aqueous liquid from the membrane. Id.
Conventional immobilized liquid membranes also inherently suffer from their hydrophilic composition, for when a gaseous component is removed from a feed gas mixture that contains water, the residual feed gas mixture becomes supersaturated. Water condenses on the hydrophilic membrane, and floods it. Thus such immobilized liquid membranes require very careful humidity control. Id. at p. 1128.
Furthermore, as disclosed in U.S. Pat. No. 4,119,408, it requires specialized process steps to replenish liquid loss in a conventional sandwich structure immobilized liquid membranes in an online gas separation system. However, the ultrathin immobilized liquid membrane of the present invention may be easily replenished from the downstream side to compensate for any liquid loss.
U.S. Pat. No. 3,625,734 discloses an immobilized liquid membrane comprising aqueous polyethylene glycol supported on a porous, inert backing membrane having deposited thereon a non-wetting microporous film of particles of polytetrafluoroethylene. To deposit the immobilized liquid film of polyethylene glycol directly on the polytetrafluoroethylene coated backing membrane, the membrane is made wettable by spraying it with a dilute aqueous solution that contains hydroxymethyl cellulose. Such an immobilized liquid membrane is thus difficult to prepare and will suffer from problems such as liquid expulsion at even low applied pressure differences as well as from liquid membrane flooding on the feed side.
Japanese Patent Publications No. 52123/1981 discloses that porous, hollow polypropylene filaments can be made hydrophilic by immersing the filaments in ethanol and then passing water through the filaments. The fibers are used to filter fine particles from aqueous solution. This reference does not mention immobilized liquid membrane technology, and does not suggest immobilizing aqueous solutions within the porous, hollow polypropylene fibers.
The disadvantages of the conventional immobilized liquid membranes are overcome by the present invention. First, once the aqueous membrane is immobilized within the hydrophobic microporous support, it is not expelled under a substantial positive pressure difference, e.g. 175 psig, applied across the membrane. Such positive pressure difference stability is wholly unexpected in a single-ply immobilized liquid membrane.
This high positive pressure difference stability means that if the microporous hydrophobic support can support mechanically the pressure difference in any given application, the conventional sandwich structure is not needed, and the immobilized liquid membrane of the present invention can stand alone. This ability to function without support provides for ease of service and cost-effectiveness as compared to conventional immobilized liquid membranes.
Second, if condensation of water due to supersaturation of the feed gas mixture occurs, the liquid membrane immobilized in the hydrophobic microporous support of the present invention will not flood. Also, if the aqueous liquid membrane is immobilized within hydrophobic microporous hollow fiber, it is stable at significant levels of applied pressure difference, especially if the higher pressure exists on the outside of the hollow fiber.
To maximize flux through a membrane and reduce the area required for a given separation, it is generally preferable to utilize as thin a membrane as possible. The present invention thus also includes a method of preparing an ultrathin, single-ply immobilized liquid membrane. Specifically, the thickness of the aqueous liquid membrane immobilized in the microporous hydrophobic support is reduced by partially removing, e.g., by evaporation, the aqueous liquid in the membrane. As the support is hydrophobic, the remaining aqueous liquid does not migrate to dried sections of the support. The ultrathin immobilized liquid membranes of the present invention have a thickness of less than about 0.0084 mm.
Therefore, it is an object of the present invention to provide a single-ply immobilized liquid membrane comprising an aqueous liquid membrane immobilized within a hydrophobic microporous support, and a method of preparing such an immobilized liquid membrane.
It is also an object of the present invention to provide a single-ply immobilized liquid membrane stable at substantial positive pressure differences applied across the membrane.
It is also an object of the present invention to provide a single-ply immobilized liquid membrane that is easy to service and relatively inexpensive to fabricate.
It is also an object of the present invention to provide an immobilized liquid membrane that resists flooding.
It is also an object of the present invention to provide an aqueous liquid membrane immobilized within hydrophobic microporous hollow fiber with the immobilized liquid membrane being stable to high levels of applied pressure difference.
It is also an object of the present invention to provide an ultrathin, single-ply immobilized liquid membrane, and a method of preparing such a membrane.
It is also an object of the present invention to provide an ultrathin, single-ply immobilized liquid membrane that exhibits superior separation ability.